Explaining variations

I have two sets of data. The first is the observed value for a number of objects, the second is the predicted value for those objects. I want to know how much, in %, my equation is able to predict. What's the name of the statistical test to know that ?

I have two sets of data. The first is the observed value for a number of objects, the second is the predicted value for those objects. I want to know how much, in %, my equation is able to predict. What's the name of the statistical test to know that ?

What "equation" are you talking about? Are you under the impression that this question makes any sense at all?

Then, you have an equation to predict the size on those animals. So you have another "vector" with predicted size P;

(P1, P2, P3 ... Pn)...

In an article, the author said his equation (the one he used to get P1, P2...) explains X% of the variation in size, but he doesn't give any clue to the method he used to find that %. I just want to know what's the name of the method he used, how can you know how much the predicted size explains the variations in the observed size ?

U have an equation y=f(x), to find the size of the animal.
Now to find its accuracy, u take a sample of size n (i.e. ur observed value) n try to find the % of variation by comparing the values of actual n observed one…
One simple method is
Find out which distribution the random variable X (the size) follows.
If it follows normal distribution, then try to estimate the mean n variance of it by using some unbiased estimator (for any other distribution, estimate the unknown parameter of the PDF).
Hope u can do the needful… good luck